CC BY is not a copyleft license. Copyleft is explicitly about the requirement to use the same license for modifications, which is present in CC BY-SA, but not in CC BY.

CC BY is recognized by the FSF as a free license and by Debian as DFSG-free. While it's not listed by the OSI as open-source, I think that's mostly because it's not a code license. So I don't think that calling it a open source license is misleading.

Of course, for open source icons I would actually expect the source files to be open...